This invention relates to a navigation system for a mobile unit such as a car navigation system, and more particularly to a GPS receiver and a GPS reception method wherein GPS (Global Positioning System) position measurement can be performed in a short time.
The GPS system is a position measurement system developed to allow a mobile unit such as an aircraft or a ship to determine the position on the earth or the velocity of the mobile unit on the real time basis utilizing GPS satellites which orbit around the earth. Recently, the GPS system is utilized widely in the field of the static survey for measuring the distance or the direction between different spots on the earth and like fields in addition to the position measurement by a mobile unit. In order to utilize the GPS system, a GPS receiver for receiving radio waves radiated from GPS satellites is used.
FIG. 10A shows a general construction of a GPS system used popularly, and FIG. 10B illustrates a conventional GPS position measurement operation. Referring first to FIG. 10A, a spread spectrum signal of 1.57542 GHz is transmitted from a GPS satellite 200. An antenna section 211 of a GPS receiver 210 receives the transmitted signal after a propagation time which relies upon the distance between the GPS satellite 200 and the GPS receiver 210. The signal received by the antenna section 211 is down converted into a signal of a predetermined intermediate frequency by a radio frequency (RF) section 212 and then supplied to a signal synchronizing demodulation section 213. The intermediate frequency signal is despread into demodulated data by the signal synchronizing demodulation section 213. The demodulated data is used for position measurement calculation by a signal processing section 214. In this manner, the signal transmitted from the GPS satellite 200 is received and used for position measurement calculation by the GPS receiver 210.
FIG. 10B illustrates a conventional GPS position measurement operation performed by the GPS receiver 210. First, when power supply to the GPS receiver 210 is made available, a frequency search is performed. The frequency search is performed in order to synchronize a frequency of a low accuracy produced by a frequency oscillator 215 in the GPS receiver 210 and having an error with a signal frequency of a high accuracy transmitted from the GPS satellite 200. If some correlation between the frequencies is detected, then the GPS receiver 210 perform a PLL (Phase Locked Loop) operation for adjustment in phase to synchronize the internal frequency fully with that of the signal from the GPS satellite 200. Then, after an edge at a bit boundary is detected and therefore data can be fetched, time information is confirmed. In particular, the TOW (Time Of Week) placed in the second word of a subframe in a hierarchical navigation message and representative of a signal time within one week in the period of 6 seconds is fetched. After the TOW is fetched, position measurement calculation is started. After the position measurement calculation is completed, position measurement data is outputted, and the current position is calculated finally.
In this manner, the GPS position measurement method requires the frequency oscillator 215 for capturing a signal from the GPS satellite 200, and in order to establish synchronism with a signal frequency of a high accuracy transmitted from the GPS satellite 200, it is required that the frequency oscillator 215 is high in accuracy. However, the oscillation frequency of the frequency oscillator 215 is fluctuated generally by a temperature or a secular change. This fluctuation prevents easy capture of the signal from the GPS satellite 200 through the use of the frequency oscillator 215, and therefore, a scheme of a frequency search must be provided separately. Since the frequency search usually requires much time, a considerably long time is required until the current position is calculated finally.
Further, in the conventional GPS position measurement method, the time required after the signal from the GPS satellite 200 is captured until all of absolute time information included in the signal is acquired is approximately 6 seconds even upon hot starting, with which the time is minimized, and in the best conditions, but usually, a time of tens and several seconds is required. Further, since position measurement calculation is performed using the acquired absolute time information, a considerably long time is required until the current position is calculated.
Furthermore, in the conventional GPS position measurement method, when position measurement is performed again after a time longer than a fixed interval of time elapses, time for fetching a navigation message newly is required. Therefore, a considerably long time is required until the current position is calculated.
Where much time is required for GPS position measurement from such reasons as described above, for example, in a car navigation system, the current position cannot be discriminated immediately after power supply is made available. This raises a problem that the route to a destination cannot be discriminated rapidly or the current position is unsettled due to an error of the self-contained navigation and this increases time until the correct position is discriminated. Further, in an apparatus of the type wherein a GPS receiver is built in or connected to a recent portable information terminal, where it is tried to use the apparatus principally during walking of the user, since the current position cannot be discriminated rapidly, the user must wait at a place with the apparatus held in hand until the position measurement is completed, which is very inconvenient.
On the other hand, also it is a possible idea to perform position measurement with power supply normally kept on. However, this causes the apparatus to consume very much power. Where the apparatus is particularly limited in power consumption like, for example, a car navigation system or a portable navigation system, it is not preferable to normally keep the power supply on.
It is an object of the present invention to provide a GPS receiver and a GPS reception method wherein GPS position measurement can be performed in a short time without the necessity to wait for periodical time information from a GPS satellite.
It is another object of the present invention to provide a GPS receiver and a GPS reception method wherein power consumption is minimized also with a minimized position measurement time.
In order to attain the objects described above, a GPS receiver to which the present invention is applied holds a frequency accuracy, a time accuracy and a navigation message and repeats its startup and standby taking a cumulative increase in error while it remains in a standby state into consideration. In particular, according to an aspect of the present invention, there is provided a GPS receiver, including reception means for receiving time information transmitted from a GPS satellite, holding means for holding the time information received by the reception means, standby mode setting means for setting the reception means to a standby mode, and start condition setting means for starting up the reception means after a predetermined time elapses after the reception means is set to the standby mode by the standby mode setting means.
With the GPS receiver, the time required for position measurement thereof can be reduced without waiting for periodical time information from a GPS satellite. Further, also where the position measurement time is reduced, the power consumption of the GPS receiver is reduced.
Preferably, the standby mode setting means disconnects power supply to set the reception means to the standby mode, and the start condition setting means starts up the reception means based on an interval within which the time information held in the holding means can keep a predetermined time accuracy. This advantageously allows repetition of startup/standby of the GPS receiver while the error of the time information remains within an allowable range.
It is to be noted that the reception means may receive a frequency of a high accuracy together with the navigation message from the GPS satellite and the holding means holds a difference in frequency between the navigation message and a frequency oscillator built in the GPS receiver as an offset in addition to the time information.
According to another aspect of the present invention, there is provided a GPS receiver, including an antenna for receiving a signal having a signal frequency of a high accuracy from a GPS satellite, a GPS block including a frequency oscillator for generating a frequency and operable to use the frequency from the frequency oscillator to read orbit information placed in the signal from the GPS satellite in synchronism with the signal and enter itself into a standby mode, and a GPS control block having a timer function for rendering the GPS block after entered into the standby mode into an activated mode based on the timer function.
Also with the GPS receiver, the time required for position measurement thereof can be reduced without waiting for periodical time information from a GPS satellite. Further, also where the position measurement time is reduced, the power consumption of the GPS receiver is reduced.
Preferably, the GPS receiver further includes a memory for storing a difference between a signal frequency of the signal from the GPS satellite and a prescribed synchronizing frequency as an offset, and wherein the GPS block updates the offset stored in the memory when the GPS block is caused to enter the activated mode by the GPS control block. The GPS receiver is advantageous in that a normally updated offset value is stored in the memory and, for example, immediately after power supply to the GPS receiver is made available by a user of the GPS receiver, an accurate current position of the GPS receiver can be displayed. In particular, when a predetermined time elapses after the power supply is disconnected, the oscillation frequency of the frequency oscillator is fluctuated, and this fluctuates the offset which is a difference between the oscillation frequency and the prescribed synchronizing frequency of a demodulator of the GPS receiver. In the a GPS receiver, however, since the memory normally holds an updated offset value, an accurate current position of the GPS receiver can be displayed immediately after power supply to the GPS receiver is made available.
Preferably, the GPS block transmits a setting signal indicative of a time interval until the GPS block is to be started up subsequently to the GPS control block, and the GPS control block transmits the standby signal to the GPS block after the GPS control block receives the setting signal from the GPS block. This allows the GPS control block to perform the control of startup/standby of the GPS receiver and thus provides an advantage in that such a trouble that the GPS block cannot be started up any more or cannot stand by any more can be prevented.
According to a further aspect of the present invention, there is provided a GPS reception method for a GPS receiver, including the steps of receiving a hierarchical navigation message from each of a plurality of GPS satellites, storing the received navigation messages into a memory, repeating startup and standby of the GPS receiver within a predetermined time to perform position measurement of the GPS receiver from the GPS satellites to update the navigation messages stored in the memory, and outputting, immediately after power supply to the GPS receiver is made available by a user of the GPS receiver, a current position of the GPS receiver based on the navigation messages stored in the memory without performing reception of the navigation message from any of the GPS satellites.
Also with the GPS reception method for a GPS receiver, the time required for position measurement of the GPS receiver can be reduced without waiting for periodical time information from a GPS satellite. Further, also where the position measurement time is reduced, the power consumption of the GPS receiver is reduced.
It is to be noted that, within the predetermined time within which startup and standby are repeated, the standby time is determined preferably taking, for example, deterioration of the accuracy of data, reduction of the number of visible satellites and so forth into consideration.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.